Ashrae Journal - December 2008 - 56

vibration, cavitations, etc., if the plant or machinery operates at its best efficiency, the noise produced is less. Material Compatibility Defense standards provide guidelines when selecting the material of construction for naval refrigeration plant and chiller packages for the marine environment. The material in contact with corrosive seawater can be gunmetal, cupro-nickel, nickel-aluminum bronze (NAB), titanium, etc. The mechanical properties of these metals such as malleability and ductility determine its application in materials like tubes, plates, and supports, etc. Generally, for seawater cooled condensers, a Cu-Ni (70:30) material is used. Also, seawater pipelines can be made of Cu-Ni (90:10). These materials are expensive and intelligent design will minimize its content in the plant. As a general design guideline, use these materials only for the seawater-contacting surface and for the other assembly use different suitable material (condenser water heads, tube sheets, and seawater tubes). Depending upon applications and regulations, certain navies prohibit use of some materials, e.g., mercury-based thermometers, asbestos-based gaskets, magnetic materials in instruments and controls, etc. System Design Optimization The main aspects that need to be considered when designing refrigeration plants and chiller packages for naval application have been covered here. A few additional parameters such as interchangeability, flexibility of operation, and ease of maintenance, etc., also must be considered. A proper system design and equipment selection can reduce energy consumption dramatically. Refrigeration system performance depends upon the design of an individual component in the system. The system is only as strong as its weakest component. The system design and equipment selection are cases of optimization influencing fixed and operation costs. Refrigerant Compressor. The compressor is the heart of the vapor compressor refrigeration system. The refrigeration capacity, evaporating temperature, condensing temperature, and refrigerant type determines the compressor type used. Energy consumption, noise and vibration limits, size, weight, and system type also govern the compressor selection. Refrigeration systems are always sized slightly more than peak load. During seasonal changes or daily load fluctuation, there is always a possibility of running the compressor on part load. The compressor efficiency decreases at part load, so many compressors have built-in capacity control arrangement with solenoid valves. Also, the power factor of induction motors drops when it runs on part load. This results in higher energy consumption. To counter this, separate multiple refrigeration systems are used to take maximum load during normal working conditions. On warships the refrigeration plant has a capacity in the range of 3.5 to 50 kW (1 ton to 14 ton). Evaporating temperatures are in the range of –20°C to –30°C (–4°F to –22°F) and 56 ASHRAE Journal condensing temperatures are in the range of 38°C to 43°C (100°F to 109°F) with R-22/R-134a as a refrigerant. The most common choices for designer for such duty conditions are open-type reciprocating compressor, scroll compressor or hermetic compressors. For refrigeration plants, peak load occurs when fresh product is loaded in the compartments. Therefore, multiple refrigeration systems are provided. During peak load multiple refrigeration systems can be operated to bring down the temperature of products to storage temperature. Thereafter, a single refrigeration system can be operated to maintain the temperature. To maintain different temperatures inside cold rooms, evaporator pressure regulating valves are provided on individual suction lines of higher temperature rooms before the common suction header. The capacity of the chiller package lies in the range of 30 to 2,700 kW (8.5 ton to 768 ton). Chilled water outlet temperatures are 6°C to 8°C (43°F to 46°F) and condensing temperatures are 38°C to 43°C (100°F to 109°F) with R-22/R-134a as a refrigerant. Open-type reciprocating compressors, screw compressors, semi-hermetic, and centrifugal compressors are better suited for such conditions. In air-handling units, the chilled water is used for conditioning air. Suitably designing AHUs and controlling airflow rates can achieve the different temperature requirements for various compartments. Motor. Motors are the prime movers for compressors and pumps. Both ac and dc motors are used for marine applications. In surface ships and nuclear submarines, ac motors are used because ac supply is available on these vessels. However, lack of ac supply on non-nuclear submarines makes dc motors mandatory. Generally, air-cooled motors are used for naval application. Seawater-cooled motors are also used. To limit the starting current value, typical ac motors for naval use are started with autotransformer or star/delta starters. Specification of motors and starters for naval ships are required to comply with standard EED-Q-071 (R3).6 All motors are required to be provided the following protections: single phasing, over-current, under/over voltage, thermal, no-load operation, moisture sensing, and short circuit. Using totally enclosed fancooled (TEFC) motors with enclosure protection of IP55/IP56 is common for naval duty. If not tested earlier, a motor is required to be type tested as per specification in the standard EED-Q-071 (R3). Environmental tests such as vibration, high temperature, damp heat, drip proof, mold growth, bump, and shock impact are required to be performed as per JSS 555557 standard. EMI/EMC test is required to be performed as per MIL-STD-461D.8 Condenser. Shell-and-tube type condensers with refrigerant on shell side and seawater on tube side are common for marine applications. Using the latest software with TEMA and ASME standards helps achieve significant saving in condenser manufacturing, as well as operating cost. When preparing the general arrangement and layout drawings, it is important to provide enough space for maintenance and tube cleaning of condenser. Thermostatic Expansion Valve. The selection of thermostatic expansion valves is particularly important because it ashrae.org December 2008

Ashrae Journal - December 2008

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